Photodiodes based on PN junctions suffer from dark current, which is essentially the normal PN junction reverse bias leakage current that will be detected in the absence of a light signal (See, FIG. 1). The dark current must be subtracted from the photodiode signal in order to sense the light signal intensity accurately. This sets the low-light limit of sensitivity of a photosensor product, particularly at high temperature where PN junction leakage is the highest.
Most photodiode processes use a single junction. In such a configuration, the dark current is the same as the reverse-bias leakage current of a simple pn junction diode. Specifically, the pn junction collects minority carriers that are thermally excited within a diffusion length of the junction.